This invention relates to hydronic radiant heating systems and particularly to high temperature shock protection for the system hydronically heated radiating body and low temperature shock protection for the system boiler. Examples of uses of the invention include hydronic radiant heating systems for melting snow and ice from outdoor drives, walks, stations, landing areas, tracks, playing fields, etc.
In any hydronic heating system the boiler return water temperature should not be so low (cool) that it causes flue gas to condense on the boiler tubes or cast iron sections. The boiler flue gas condensation point is typically about 130.degree. F., and at and below that temperature the flue gas condenses and corrodes the outside of the tubes or cast iron sections and the flue gas stack. This is referred to herein as boiler "cold shock".
At the water high temperature end in an hydronic heating system, heat from the hot supply water from the boiler is fed to the heated body can deliver too much heat to the body so that the body is damaged or too hot for comfort. This can happen whether the heated body is a radiator, a conductor of heat to a workpiece or a heat exchanger that heats water fed to an outdoor slab to melt snow and ice on the slab. In the latter case the slab can be at outdoor ambient temperature that may be below zero and the slab and/or the slab heating system can be damaged. This is referred to herein as radiator, conductor (workpiece) or slab "hot shock".
Hydronic snow and ice melting (HSM) systems yield savings by reducing the costs of snow removal and, important for business and commercial establishments, is the investment payback by reduced building cleaning costs through the elimination of salt and sand and carpet damage from tracked in salt and sand. It is estimated that carpet replacement can be reduced to a third as frequent. Above all, slip and fall accident liability around the exterior of the building is dramatically reduced. On hospital emergency entrances and exits, loading docks, ramps, packing areas, helicopter pads and truck scales the installation of a snow and ice melting system is also a safety and accessibility issue. For football and soccer fields, and greyhound race tracks the payback can come from a few saved events.
Automated snow melting systems using electrical heating elements embedded in a concrete slab have been used and are less expensive than hydronic (forced hot water) systems, but have life spans tending around only 10-15 years. Once they break, they're gone. Maintenance and repairs are prohibitive when it comes to breaking open concrete to get at the heating elements.
Hydronic snow melting (HSM) systems cost more than electrical for initial installation, but they are more durable, based on evidence from European and U.S. installations, and operating costs are much lower. In particular, electronically cross-linked polyethylene (PEX) tubing embedded in the slab used widely in Europe and now in the U.S., is guaranteed for 30 years and, like all such warranties, these are conservative estimates of system life. The idea is to build a system that will last a lifetime, or at least outlast the slab in which it is embedded.
A large HSM installation can use over 100,000 lineal feet of 20 mm PEX tubing to cover 90,000 square feet of driveways, walkways, ramps and helicopter landing pad and require over 20 million Btu/hr boiler capacity.
The boiler size for an HSM system is based on BTU/hour/square foot of snow melt area depending on the geographic location of the system; and snowfall, design temperature and wind velocity are all factors. Experience shows that output requirements can range from 150 to 300 BTU/hour/square foot, depending on location. It is most important that the boiler system be of adequate size for the HSM system If the needed BTU's are not available during design conditions the system will not work when needed most.
Insulation beneath the snow melting tubing should be used whenever possible. One inch foam board is sufficient. The tubing should be a suitable thermoplastic material which will stand up to the extreme fluctuations of water temperature cycling and weather conditions. The preferred tubing material with the greatest life expectancy for this application is polyethylene cross-linked electronically, such as the PEX tubing, or cross-linked chemically by the Engel method. The tubing size for most applications is 5/8 inch inside diameter with each HMS slab system circuit (loop) not exceeding 300 ft. Larger tubing diameters are used when longer loops are required Anti-freeze solution of 30% to 50% depending on design temperatures, is mandatory. The non-toxic type anti-freeze is recommended to avoid any potential environmental damage.
Control strategies are numerous depending on system requirements.
a) Intermittent system activation upon snow and ice conditions. PA1 b) Minimum surface temperature maintenance at all times with temperature step-up when conditions call for it. PA1 c) Constant temperature maintenance for soil frost removal in sports facilities. Energy consumption varies greatly depending on the selection of any of these three. PA1 1) a pilot status, where all the switches are on and ready to go from a cold start; or PA1 2) an idle mode, in which the system is kept heated to slightly below the freezing mark at all times. The pilot status conserves fuel, but has the disadvantage of slow response time. In starting up from a cold start, a system could be overwhelmed by an early season, heavy snowfall or ice build-up. Most users choose the idle option, which keeps the slab heated to 25.degree. F. to 30.degree. F. at all times, snow or shine, because it is easier operationally and on the equipment to go from 30.degree. F. to the 38.degree. F. to 40.degree. F. required for snow melting, than to try to reach it in a situation where the slab temperature may be well below freezing.
Outdoor slab surface temperature and moisture sensors are used to detect the conditions and activate the system via a programmable set point controller. However, manual system activation and deactivation can be the best method, because anticipating snowfall is the best way to overcome the system's lag time. Anticipating snowfall gives the user time to make the exterior slab surface temperature ready for the first snowflakes. Once considerable snow accumulation occurs (2 to 3 inches) before proper surface melting temperature (38.degree. F. to 40.degree. F.) is achieved, it becomes very difficult for the system to catch up. Even with sufficient boiler horsepower it can become a loosing effort, particularly when wind chill factors come into play.
Proper HSM system supply water and boiler return water temperatures are crucial to protect the boiler and the exterior surface radiator and slab. As mentioned above, the boiler must be protected from excessively cold return water, called "cold shock" and the slab must be protected from excessively hot water, called "hot shock". Some boiler room piping options and electrical controls have been used in efforts to gain these protections. Stainless steel plate heat exchangers are used with conventional steel and cast iron boilers to separate the exterior radiator system (the slab system) that contains anti-freeze in the water, from the boiler system. Low temperature copper and condensing boilers can be directly installed in the system. A boiler two-way by-pass valve has been used in an effort to add hot supply water to the cold return water and so avoid boiler "cold shock". Even the sturdiest boiler cannot withstand the thermal shock that can occur under some operating conditions.
The system may be triggered for operation by dual sensors in the concrete, responding to temperature and moisture conditions. When the concrete gets wet and cold, the sensors tell either the boiler or the valves to respond. Which one gets the signal may depend on one of two modes of operation. The entire system typically gets shut down during the warm weather months, and re-energized sometime in fall when there is potential for freezing temperatures. At that point the user has the choice of operating in either:
HSM system operation and maintenance shouldn't be any more difficult than normal space heating requirements. The boilers, burners, pumps and valves must be kept functional, but nothing that can be done to the under slab components once they are installed.
The boiler(s) of an HSM system must have high resistance to thermal shock. This points to one of the major differences between an HSM system and the very similar technology of household radiant floor heating (RFH). Whereas RFH boiler supply water gets delivered ideally at gentle temperatures in the 100.degree. F. and lower range, 120.degree. F. to 160.degree. F. is the ideal boiler supply water operating temperature range for an HSM system. In addition, an HSM system operates at about 60 psi and feeds several separate snow melting zones. Here, as in all hydronic systems, the boiler return water temperature must not be so low that the outside of the boiler tubes or cast iron sections is lower than the flue gas condensation temperature of about 130.degree. F. Below that risks corrosion of the boiler tubes or cast iron sections and the stack.
From a technical standpoint, particularly for an HSM system, this is the trickiest part of the system, because the slab system water from the heat exchanger gets exposed to very cold outdoor temperatures as it winds its way through the frigid concrete slab. The tubing is installed, ideally, two inches below the surface and 10 to 12 inch spacing between tubes. In the past, warming the boiler return water to reduce "cold shock" has been done using an electrically controlled, two position, two-way valve for diverting some boiler supply water to the boiler return water line; and at the same time, somehow cooling or diluting the boiler supply water to the 120.degree.-160.degree. delivery range so that the HSM slab system supply water from the heat exchanger is not so hot as to shock the concrete slab.
The present invention has application to many hydronic heating systems including systems that heat principally by conduction as well as radiation and including systems that heat a workpiece, RFH systems for a dwelling and particularly HSM systems, such as described in the specific embodiments, which all make use of three-way modulated, feedback controlled valve systems for reducing boiler "cold shock" and for reducing slab "hot shock".